At 4 a.m. on Hell Night, Mark Parris and 12 other wannabe SEALs were sitting up to their necks in the most vile water they had ever seen.

As a pair of frogs mated a couple of inches from Parris’s face, he was ordered to do push-ups and flutter kicks by a retired Navy SEAL named Don Shipley. With his waterlogged boots, Parris struggled along with the other men to lift his legs. And there were hours more of running, lifting and paddling ahead.

Parris, a 51-year-old electrician from Long Island, N.Y., with arthritic knees, was suddenly filled with doubt about his decision to sign up for an Extreme SEAL Experience, a week-long course that offers civilians a rare taste of what it takes to be a Navy SEAL.

The pond “was full of I don’t know what slime,” he recalled later. “There had to be snakes in there. And it smelled. I thought, ‘This disgusting pond. Why I am doing this?’ ”

Months before an elite SEAL team seized the public imagination by killing Osama bin Laden, Parris and his miserable pond mates had each agreed to pay Shipley $1,900 to push them to the limits of their physical and mental endurance. In addition to Parris, there was a gun-happy insurance account manager from Vallejo, Calif., a sandwich delivery guy from Taiwan who sounds like Arnold Schwarzenegger, and a home-schooled farmhand from South Florida.

They fell into two categories: young men who wanted to become SEALs, and middle-aged men who wanted to test their manhood.

Bin Laden’s death made their decision seem only slightly less insane to their friends and family.

“When I told my wife I was coming, she said, ‘It’s menopause,’ ” said Parris, who spent three years swimming and working with a professional trainer to prepare for the course. He lost 25 pounds in the process.

The last obstacle he faced was arthritis, which for years prevented him from walking up stairs unaided. Then, six months ago, he received three injections of a special oil-based fluid in his knees to make it easier for him to run.

“I felt like I was 20 years old again,” Parris said.

He got to test his knees during Hell Night, a 24-hour-period of nonstop physical exertion that is supposed to simulate the famous Hell Week that SEAL candidates must endure. Real SEAL school, called Basic Underwater Demolition school, lasts six months, and about 80 percent of the participants don’t make it through.

Shipley, who founded the Extreme SEAL Experience in 2006, said Hell Night doesn’t truly replicate the rigors of Hell Week. But for his customers, it’s challenging enough.

They run more than eight miles, carry a 200-pound log above their heads, get sprayed in the face with cold water and have someone scream in their ear that their “man card” is being taken away because they are “doing push-ups like a girl.”

The point, Shipley said, is to get them push past the pain. A lot of it is mental, not just physical.

The rail-thin sandwich delivery guy from Taiwan dislocated his kneecap during the first run but still made it through the rest of the night. The only guy who didn’t finish was a buff computer engineer from Richmond who threw his back out. For the rest of the week, the other guys called him “back boy.”

Rules for real men

The night after Hell Night, the men were still worn out as they gathered around a bonfire that Shipley, 50, lit with a flamethrower. (“I never use matches if I have [one],” he explained.)

Shipley asked Parris about a blister on his left foot. Parris said he’d put a Band-Aid on it.

“Real men don’t wear Band-Aids,” Shipley said, only half-kidding, “or drink from a straw.”

Later a few of them headed to a drying rack to gather their clothes for the next day’s torture.

John Speaker, a 35-year-old technology manager for a cable company in Texas, held up a still-soaked shirt. “Nothing like having clothes you haven’t washed and putting them back on,” he said. “I have a feeling they’ll be wet all week.”

The men walked back to their barracks. On the other side of the field was the “media center,” an open shed decorated with a string of red lights where “Rocky” had been playing earlier. Next to the pond was a large cabin where the rest of the men slept and all of them gathered for their meals.

Each day, the men learned crucial SEAL skills, including how to break out of leg irons, how to keep from drowning and how to jump out of a helicopter.

Unlike in real SEAL school, though, they don’t get expelled for failing. “Nobody comes here to fail,” Shipley said. “We put them back in the game. They pay to come here to succeed.”

If they get hurt and can’t finish, they get their money back. Once in a while, a few give up. Last year Shipley lost four out of 223.

In the morning, Shipley met the men in the clearing between the cabin and his house.

“Morning, girls,” he said. He was about to go through the day’s schedule when he looked down and spotted a dead mole rat by his feet. He bent down, picked it up and flung it over his shoulder.

The other main instructor, ex-SEAL Nathanael “Lalo” Roberti, who saw 11 of his fellow SEALs get killed during a mission in Afghanistan, arrived dressed like an aerobics instructor, in a T-shirt and shorts over a pair of half tights. He brought an orange mat, a beat-up SEAL training manual, his BlackBerry and a tin of dipping tobacco.

A group consisting mainly of the older students whom Shipley called the “smart guys” went with Shipley to shoot BB guns at one another, while the younger group — the “dumb guys” — went with Roberti to learn how to clear a room of the enemy.

Roberti parked himself in a folding chair. In between barking orders at the men, he said: “I try not to laugh. A lot of these guys aren’t disciplined. They play video games all day.”

Later at an old hog farm nearby, the men stormed an abandoned barn. Parris, decked out in full camouflage and a mask to protect his face from BBs, waited by a door with three other smart guys for their turn.

“We’re the A team,” one of them joked. “The aged team.”

When Roberti yelled “Bust ’em,” the men went in one by one, toting M-4 Airsoft training guns and firing at cutouts of men in head scarves pointing automatic weapons at them. Once the men hit their targets, they yelled “clear,” “all clear,” and “target secured!”

Wang, the guy from Taiwan, described it as cops and robbers, but “more intense.”

‘I feel like Superman’

SEAL stands for sea, air and land. And the next day was all about water. The men had to swim sidestroke, tie knots underwater, and learn to keep themselves from drowning.

Parris struggled with his sidestroke. But he outdid many of the younger guys during one of the most dreaded pool exercises: underwater sit-ups wearing masks that don’t allow them to see or to breathe through their noses.

The sensation of water running down his throat prompted Ethan Fiedler, a 19-year-old from Watsonville, Pa., to panic and rip off the mask.

Parris, by contrast, did his without a glitch.

“I’m definitely going back a changed man. I will probably be going back with the idea to be more driven, to get through certain hurdles knowing I went through Hell Night,” Parris said. “Now, I feel like Superman. I can do anything.”